A gravity gradiometer is disclosed in our International Patent Application No. PCT/AU2006/001269 and several concurrently filed applications. The content of International Patent Application No. PCT/AU2006/001269 is incorporated into this specification by this reference.
Gravimeters are widely used in geological exploration to measure the first derivatives of the earth's gravitational field. Whilst some advances have been made in developing gravimeters which can measure the first derivatives of the earth's gravitational field because of the difficulty in distinguishing spatial variations of the field from temporal fluctuations of accelerations of a moving vehicle, these measurements can usually be made to sufficient precision for useful exploration only with land-based stationary instruments.
Gravity gradiometers (as distinct from gravimeters) are used to measure the second derivative of the gravitational field and use a sensor which is required to measure the differences between gravitational forces down to one part in 1012 of normal gravity.
Typically such devices have been used to attempt to locate deposits such as ore deposits including iron ore and geological structures bearing hydrocarbons.
The above-mentioned gradiometer has a sensor in the form of a sensor mass which is pivotally mounted for movement in response to the gravity gradient.
The gravity gradiometer disclosed in the previously mentioned International application includes sensor bars which are mounted in a housing for pivotal movement in response to the gravity gradient about a flexure web. Transducers which include coils are provided for measuring movement of the masses by a change of inductance of the coils when the masses move closer to or further away from the coils to thereby provide a measure of the movement to in turn enable a measure of the gravity gradient tensor to be made. The sensing coils are formed from superconducting material and are operated at cryogenic temperatures. A current is initially induced in the coils and that current is changed when the inductance of the coils is changed so that the change in current provides the measure of the gravity gradient. The current is initially induced in the coils by providing a heat switch in the form of a resistor so that the superconducting circuit can temporarily be made non-superconducting by elevating the temperature of part of the circuit above the super-conducting threshold so the circuit effectively becomes open at that point. Input leads are connected to the circuit so that a current can be supplied from a current source to induce the initial current in the circuit. The resistor is then deactivated so that the part of the circuit again becomes superconducting so that the induced current simply continues to circulate through the circuit.
Typically in order to form the heat switch, part of the superconducting circuit is simply wound around the resistor so that it heated by heat generated when current flows through the resistor to thereby cause that part of the circuit to warm to a temperature above the superconducting transition of the superconducting material. The formation of the superconducting material about the resistor can be difficult and also, since the resistor is relatively large, this form of heat switch has disadvantages in gradiometers where space considerations can be of significance.